This invention relates to separation of phosphate minerals from other minerals, particular from phosphate containing ores.
World fertilizer production continues to be a crucial factor for the efficient growth of crops to feed the peoples of the earth. Phosphate is an essential ingredient of fertilizer, and the world production of phosphate rock is more than 147.1 million mt per annum as indicated in Table 1. It should be noted that the U.S. is the largest producer of phosphate rock with most of its 40-plus million mt per annum coming from the vast sedimentary deposits in central Florida. However as discussed at recent Engineering Foundation Conferences on Phosphate, December 1993 and 1998, Palm Coast, Fla., many technological problems must be solved if we are to continue to produce phosphate rock at our current rate of consumption. Unless these technological problems are solved, phosphate rock may be in short supply. The critical nature of this situation is clarified by Isaac Asimov, xe2x80x9cWe may be able to substitute nuclear power for coal power; and plastics for wood; and yeast for meat, and friendship for isolation-but for phosphorus there is neither substitute nor replacement.xe2x80x9d
One of the most important processing technologies which accounts for this significant production of phosphate is the process of froth flotation which is used exclusively around the world. The flotation process is based on appropriate surface chemistry control in order to selectively generate hydrophobic surfaces on certain mineral particles while maintaining the surfaces of other mineral particles in a hydrophilic state. The phosphate industry has relied on the flotation process since the 1950""s and will continue to do so for decades to come.
A significant problem which now faces the phosphate industry worldwide is the selective separation of phosphate minerals from carbonate minerals, particularly dolomite, by froth flotation. The Florida phosphate industry is no exception to this problem, and future production from the phosphate deposits of central Florida will require the development of new flotation technology for improved separation efficiency. Typical specifications of the phosphate rock concentrate for the production of fertilizer are:
 less than 1% MgO
 greater than 30% P2O5 
 less than 4% SiO2 
Traditionally these specifications have been relatively easy to meet because the run-of-mine phosphate rock has been mostly siliceous rock rather than calcereous or dolomitic rock. The current state-of-the-art has been discussed in the literature and at the Engineering Foundation Conferences on Phosphate, December 1993 and 1998, Palm Coast, Fla. Now, it is evident that the siliceous resources will soon be exhausted, and only the difficult-to-separate, carbonate-bearing rock will remain as our country""s major phosphate resource. Efforts have been made for some time to treat such carbonate-bearing rock, particularly the dolomitic phosphate reserves of Florida.
Froth flotation for the separation of phosphate minerals from other gangue minerals has been practiced by fatty acid flotation with pine oil as frother since as early as 1928. Many flotation strategies for the processing and concentration of phosphate ores have been developed since then. The conventional phosphate flotation process for sedimentary deposits of central Florida is the xe2x80x9cdouble floatxe2x80x9d process, viz. anionic flotation of phosphate minerals at alkaline pH, followed by cationic xe2x80x9creversexe2x80x9d flotation of silica from the initial phosphate concentrate at neutral or acidic conditions. The Florida phosphate industry, with few exceptions, still uses this standard method.
Summary of Phosphate Flotation Processes
Flotation is the most widely used method for the treatment of phosphate rock. The flowsheet design depends on the type of ore (endogenic or sedimentary deposit) and the nature of impurities (silica or carbonate) to be removed. Phosphate flotation strategies can be classified as follows:
Direct Flotation of Phosphate
The phosphate minerals are floated directly using carboxylate (fatty acids and the corresponding soaps) collectors, often co-added with hydrocarbon supplements (such as kerosene, fuel oil, etc), and appropriate reagents for gangue depression. This process is very successful for endogenic siliceous phosphate deposits. In this process the advantages are a relatively simple flowsheet, and low cost.
Reverse Flotation of Carbonate Minerals
Dolonite and other carbonate minerals are floated using carboxylate collectors under slightly acid conditions with phosphoric acid added for the depression of phosphate minerals. If the feed contains a significant amount of silica a final concentrate cannot be obtained with this strategy alone.
Double Float Flotation Processes
There are two flotation processes which fall in this category. One is the Direct-Reverse flotation process. In this process the first stage is designed to float as much phosphate mineral as is possible using carboxylate collector. In this stage some of the silica and/or carbonate gangue is rejected. The second stage is referred to as reverse flotation. In this stage only as much silica or carbonate mineral is floated from the initial phosphate concentrate as is required to meet the final desired product specifications. For example this xe2x80x9cdouble floatxe2x80x9d process is widely used in central Florida phosphate industry, viz. the anionic flotation of phosphates at pH8-9.5, followed by cationic flotation of quartz from the acid-scrubbed rougher concentrate at pH 6-7.5.
Another process that falls in this category is the Reverse-Direct flotation process. In the first stage the carbonate or silica gangue mineral is floated and then the phosphate flotation is carried out. When the feed contains two types of gangue mineral (silica and carbonate) this double stage strategy may not be efficient.
Phosphate flotation efficiency needs to be improved in several ways. Technology needs to be developed to eliminate the double flotation processes, to improve the flotation efficiency for both coarse and fine phosphate, and to solve the dolomite problem. A very important factor in flotation technology is the use of appropriate reagents. Collectors and other reagents need to be developed to improve coarse particle flotation and to achieve selectivity with respect to carbonate minerals, particularly dolomite. The development of highly selective collectors, which are specific to the surface structure of a particular mineral, is essential for the exploitation of relatively more difficult-to-process ore deposits, particularly for the separation of semi-soluble minerals having a common cation. The difficulty in the separation of phosphate from dolomite is probably due to the fact that both minerals have the same cation Ca2+, and similarly sized anions, PO43xe2x88x92 and CO32xe2x88x92.
The major dolomite problem associated with the future reserves in central Florida is found with the pebble fraction (xcx9c6% MgO). The development of a satisfactory processing strategy will probably involve grinding and classification followed by flotation or some other method to separate the dolomite from the phosphate. A number of flotation technologies have been under development for the Florida carbonate-bearing phosphate rock, and most under current study involve the anionic flotation of carbonate minerals from an acid suspension. These include,
USBMxe2x80x94pH 6.0 depression of phosphate with hydrofluosilic acid.
TVA Diphosphonic Acidxe2x80x94pH 6.5 phosphate depression with ethylidene hydroxydiphosphonic acid.
Aluininum Sulfate/Tartratexe2x80x94pH 7.5 to 8.2 phosphate depression with Al2(SO4)3/Na tartate.
Sulfuric Acidxe2x80x94pH 5.0 to 5.5 phosphate depression simply by sulfuric acid, fast conditioning and flotation time in order to maintain pH. Even better flotation separation efficiencies are obtained at pH 3.5 to 4.5.
Phosphoric Acidxe2x80x94pH 5.0 to 5.5 phosphate depression by phosphoric acid.
IMC Anionicxe2x80x94pH 5.0 to 6.0 flotation of carbonate with sulfonated fatty acid, phosphate depression with sodium tripoly phosphate.
In all of these cases, the process strategy involves flotation of carbonate minerals from the phosphate minerals in an acid circuit with anionic collectors. However, the separation efficiency has been limited by the control of pH and the effective depression of apatite or other phosphate minerals such as collophane and francolite. At this point, a satisfactory process strategy has not been established, and the dolomitic resources cannot be processed economically.
Hydroxamate collectors have had limited use in the industry for flotation separations. Examples include the separation of colored impurities from kaolin clay, the recovery of copper oxides from ores, and the selective flotation of iron oxide from gangue minerals. All of these separations involve separation based upon specific hydroxamate adsorption at metallic cationic sites of the minerals to be floated, and it is believed that the collectors function by a selective interaction with these cations of the mineral""s lattice. Hydroxamate collectors have not been found in the prior-art to be effective for separations of minerals composed of alkaline earth cations. Specifically, there is no indication or expectation in the prior-art that hydroxamates would be effective for the flotation of phosphate minerals, such as apatite, collophanite, and francolite.
Objects of the Invention
It is, therefore, an object of the invention to provide an effective, inexpensive, method for separating phosphates from ores.
Another object of the invention is a method of separating phosphate minerals from high-content dolomite phosphate ores.
Further objects of the invention will become evident in the description below.
In the present invention hydoxamates are used as selective collectors for the flotation separation of phosphate minerals from dolomite. Although hydroxamates have been used for more than twenty years as flotation collectors, their use for phosphate flotation has been ignored since phosphate flotation involves the flotation of calcium phosphate minerals and hydroxamates are not expected to adsorb at calcium surface sites. For this reason, the prior-art of flotation would teach one of ordinary skill away from the use of hydroxamates for the flotation of phosphate rock.
In general, alcoholic solutions of alkyl hydroxamates and other hydroxamates are added to a particular suspension of phosphate ore and after aeration the phosphate minerals are floated from the gangue minerals, such as dolomite. It is understood that the general use of the term hydroxamates includes the acid form, allylhydoxamic acid.
The present invention comprises a method for separating phosphate minerals from a mineral mixture, such as a phosphate ore. The mineral mixture is conditioned by contacting in an aqueous in environment with a hydroxamate collector in an amount sufficient for promoting flotation of phosphate minerals. The mixture is then subject to flotation conditions to float the phosphate minerals and separate them from the gangue minerals, such as quartz, calcite, and dolomite. The collector comprises a hydroxamate compound of the formula; 
wherein R is generally hydrophobic and chosen so that the collector has solubility or dispersion properties such that it can be distributed in the mineral mixture. M is a cation, such as hydrogen, an alkali metal, or an alkaline earth metal, and is chosen such that the collector has solubility or dispersion properties such that it can be distributed in the mineral mixture. The collector is distributed in the mineral mixture by any suitable method. The conditioning in the present invention can be accomplished using traditional conditioning systems, and the process of the invention can be used directly in existing plants with little or no equipment modification.
In a preferred embodiment of the invention, the collector also includes in addition to the hydroxamate an alcohol of the formula;
Rxe2x80x2xe2x80x94OH
where Rxe2x80x2 is the same or different from the R of the hydroxamate, and is generally hydrophobic and chosen such that the collector has solubility or dispersion properties so it can be distributed in the mineral mixture.
Hydroxamate and alcohol compounds having suitable properties for use in the invention are believed to include those where the R or Rxe2x80x2 is an alkyl, aryl, or alkylaryl group having 6 to 18 carbon atoms, and M is typically hydrogen, an alkali metal or an alkaline earth metal.
It should be noted, that hydroxamic acid can exist in two forms, i.e., the N-acyl derivative (I) or the O-acyl derivative (II), as shown by the following structures; 
while the N-acyl form is the most common. It can exist in either of two tautermeric forms: 
The formula used herein and in the claims, shows the N-acyl hydroxamate form, but it is understood that O-acyl form, as well as the tautermeric forms of N-ocyl are contemplated by the invention and included within the general definition recited in the claims.
Other chemicals and reagents, such as dispersants and modifiers, may optionally be added to the mineral mixture. These may be desired with certain mineral mixtures to, for example, aid dispersion or modify the pH. Examples include sodium carbonate, and sodium silicate.
The present invention is particularly useful for separating phosphate minerals from dolomite, calcite, quartz and other common gangue minerals. The separation efficiency of the hydroxamate collector is unexpected because previous flotation separations using hydroxamates have been based upon adsorption at metallic cationic sites on the mineral surface, for example, the flotation of iron minerals from kaolin. Since dolomite and phosphate minerals usually have the same cation, calcium, it would be expected that any differentiation would be at most insignificant, as demonstrated in the literature, for example in S. M. Assis, et al. xe2x80x9cUtilisation of Hydoxamates in Minerals Froth Flotationxe2x80x9d Minerals Engineering, Vol. 9, No. 1, pp. 103-114, 1996.
However, unexpectedly in the present invention, differentiation is achieved. Not only is the magnitude of the differentiation unexpected, but its very existence is unexpected. Separations with hydroxamate in the prior-art have involved hydroxamate adsorption at cationic sites of heavy metal minerals, particularly transition-metal cations and rare-earth metal cations. These from very stable complexes with hydroxamate, which makes hydroxamate an effective flotation agent for such minerals. The affinity of hydroxamate for the calcium cationic, in comparison is much weaker, and complexes of hydroxamate with calcium are much less stable. This is evident by consideration of the stability constants shown in Table 2, which shows that the stability constant (K) for the calcium hydroxamate is orders of magnitude lower than any of the other metal cations listed. Thus, it is unexpected that hydroxamates, which form such weak calcium complexes could be effective in selectively floating a calcium-cation phosphate mineral
[1], [2] [3]
[1] Pradip and Fuerstenau, D. W. Mineral flotation with hydroxamate collector. Reagents in the Mineral Industry, (M. J. Jones and R. Oblatt eds), London, UK, 161(1984)
[2] Schwarzenbach G. and Schwarzenbach K. Hydroxamatekoplexe I. Die Stabilitat der Eiser (III)xe2x80x94Komplexe einfacher Hydroxamsauren und der Ferrioxamins B. Helv. Chim. Acta, 46, 1963, 1390-408
[3] KB Quast, Flotation of hematite using hydroxamates as collector, Reagents in the Mineral Industry, (M. J. Jones and R. Oblatt eds), London, UK, 161(1984)
The stability of hydroxamate complexes arises due to chelation phenomena as indicated by the following structure for a divalent metal cation Mxe2x80x2. 
It is evident that the calcium complex is the least stable complex and in this regard it is unexpected that hydroxamate serves so efficiently and with such great selectivity as a collector for phosphate minerals. (See J. W. Munson, Chemistry and Biologic Activity of Hydroxamic Acids An Overview, Chemistry and Biology of Hydromamic Acids, Editor: Horst Kehl, Kirksville, Missouri, 1982.)
By practice of the present invention it is possible to solve the dolomitic phosphate problem as well as provide new technology for the processing of the traditional phosphate resources. The significance of this discovery is revealed by the flotation data presented in FIG. 1. It is evident that excellent selectivity is possible with hydroxamates. Specifically apatite recovery of more than 95% is possible by single-stage flotation with a collector addition of 250 g/ton, whereas flotation of dolomite and quartz is not possible even at hydroxamate additions of up to 100 g/ton.
Potential Benefits
The successful development of the new flotation technology for dolomitic/calcitic phosphate ores will allow for the utilization of many additional phosphate resources. It is estimated that almost 20 million tons of phosphate rock were produced in the U.S. by froth flotation in 1997 and reagent demand was substantial, corresponding to almost 50 million dollars. During the next decade it is expected that a significant portion of this production will have to come from dolomitic/calcitic phosphate ore. Already such ores are processed rather inefficiently in Utah and Idaho. A similar situation exists with the North African and Middle Eastern deposits. It is expected that the flotation chemistry of the present invention will provide for a more efficient separation.